Touchscreens have been used for some time for user interfaces to computers and computer-based systems. They offer advantages over other user interface hardware such as keyboards and other remote, non-graphic devices, e.g., the "mouse." The touchscreen eliminates external hardware, is simpler and more intuitive to use, and thus reduces expense, space requirements, maintenance, and the need for user training. The touchscreen permits direct association between the screen display and user input, and as such is more "user friendly" than external keyboards and other remote devices.
A particularly popular application for touchscreens is in point-of-sale (POS) systems in retail stores and restaurants. POS systems incorporate order taking, cash register, inventory control and other functions. The POS display can be readily updated to reflect changes in inventories, menus and prices. The touchscreen is easy to use, permits quick and efficient order taking and cash register functions, and requires little user sophistication or technical knowledge.
Touchscreens have been implemented in several technologies, including those using infrared sensors, capacitive contacts and resistive membrane overlays. A relatively new touchscreen system known in the art employs surface acoustic wave (SAW) technology, which offers several advantages over other technologies. A SAW touchscreen requires only a thin, transparent panel, which may be glass, over the display and is desirable in display applications where brightness, contrast and packaging are critical. SAW touchscreens can also provide a third coordinate, or z-axis, in addition to the conventional x-y coordinates. This input is proportional to touch pressure and provides enhanced software flexibility.
Surface acoustic waves are mechanical waves which propagate in the surface of the medium in which they are generated. SAW touchscreens utilize high-frequency acoustic waves generated by transducers which travel over the surface of the screen at a precise, known speed. These waves are reflected by arrays of reflectors located on the perimeter of the screen to provide a scan of the x and y coordinate axes of the screen. When the screen is touched, the touching device (e.g., a finger or stylus) absorbs a portion of the SAW energy, attenuating the x and y signals. By measuring the time at which the attenuation occurs on each axis and comparing the position of the attenuation relative to the scanning signal envelope, the location of the touch can be calculated.
The touchscreen itself may be a glass plate which is mounted in a housing over the face of a display panel such as a CRT, LCD, gas plasma or other type of display. The touchscreen plate should be evenly supported around its edges. The housing contains the electronic hardware necessary to drive the display and operate the touchscreen. It is desirable to protect the display, internal electronics, transducers and reflective arrays from contamination by moisture and foreign matter. This is particularly true in applications such as restaurants where there is a likelihood that food and beverages may from time to time be spilled on the touchscreen.
A need exists for an effective and useful liquid impermeable seal between the touchscreen and the housing. Soft durometer materials such as caulking, although capable of providing such a seal, absorb significant amounts of SAW energy so as to render the touchscreen inoperable. Such soft durometer materials that can provide a liquid impermeable seal not only impair the propagation of waves across the touchscreen, but also acoustically couple the screen and can cause a false touch to be registered around the entire perimeter of the screen. On the other hand, known sealing materials of a hard durometer do not absorb SAW energy sufficiently to trigger a false touch, but also do not provide a liquid impermeable seal because liquids leak between the seal and the glass by capillary action.